Glad to help!
1) I don't understand. "flip" was removed from Spine in 3.0.00 (replaced with negative scale). It has not been changed since. We do not make such behavioral changes lightly, but removing flip was necessary. Moving from 3.7 to 3.8 should not change any of your skeletons or animations.
2) Are you sure you have the same axes selected (local, parent, or world) in both cases? If so we'll need steps to reproduce this so we can see the problem. For example, maybe you can tell us what steps to take to see the problem with the spineboy project, or you can email steps to take using your project file: contact@esotericsoftware.com
3) Compatibility is done at the Spine project file level. Newer versions of Spine can always open older Spine projects (ie Spine has backward compatibility). Older versions of Spine cannot open newer Spine projects (ie Spine does not have forward compatibility).
Spine does not have any compatibility for JSON or binary data: you should always use the same Spine version that matches the JSON or binary data you are importing. Spine does not support opening JSON or binary data from other versions of Spine. In practice sometimes it will work, but this is a coincidence and not officially supported.
You should not keep JSON or binary data and expect to import it into newer versions of Spine. Instead, keep the Spine projects which can always be opened by newer versions of Spine.
If you do have old JSON or binary data, to bring it into a newer Spine version you'll need to first import it into the old version of Spine, save a project file, then open the project file with the newer version of Spine.
